1. Field of the Invention
The present invention relates to a crank press and a control method thereof, more particularly to a crank press with a dual protection mechanism and a control method thereof.
2. Description of the Related Art
Referring to FIG. 1, a conventional punch device 900, which is provided with a hydraulic overload protector, of a press machine is illustrated. The punch device 900 includes a slider 90, a hydraulic tank 91 disposed in the slider 90, a cover block 92 disposed at a lateral wall of the slider 90, a hydraulic pump 93 disposed on the cover block 92, and an overload controller 94. The slider 90 is formed with an opening 901 which communicates fluidly with the hydraulic tank 91. The cover block 92 is disposed to cover the opening 901, and a hydraulic outlet 921 is formed through the cover block 92 and communicates fluidly with the opening 901. The hydraulic pump 93 pumps hydraulic oil out of the hydraulic tank 91 and sends the hydraulic oil into a hydraulic cylinder 95 which is located above the hydraulic tank 91 via the hydraulic outlet 921. The overload controller 94 detects pressure in the hydraulic cylinder 95.
When the punch device 900 punches a workpiece (not shown) and receives a reaction force therefrom, the hydraulic oil in the hydraulic cylinder 95 is compressed so that pressure of the hydraulic oil is changed. At this moment, when the overload controller 94 detects that the pressure in the hydraulic cylinder 95 exceeds a predetermined pressure, the overload controller 94 controls the hydraulic pump 93 to stop pumping and sending the hydraulic oil to the hydraulic cylinder 95, or to release the hydraulic oil from the hydraulic cylinder 95 so as to prevent an excessive pressure in the punch device 900. On the other hand, a control system of the press machine may drive a clutch to disengage a power connection between the punch device 900 and a servo motor of the press machine, or may activate a brake for achieving an effect of overload protection.
However, the control mechanism of the conventional hydraulic overload protector has the following drawbacks.
1. Oil pressure change is unstable such that the control mechanism may not be implemented with ease.
2. The punch device 900 is protected passively only when overload happens, and may not be protected actively.